One of the problems faced by the perfumery industry lies in the relatively rapid loss of olfactive benefit provided by odoriferous compounds due to their volatility, particularly that of “top-notes”. In order to tailor the release rates of volatiles, delivery systems such as microcapsules containing a perfume, are needed to protect and later release the core payload when triggered. A key requirement from the industry regarding these systems is to survive suspension in challenging bases without physically dissociating or degrading. This is referred to as performance in terms of stability for the delivery system. For instance, fragranced personal and household cleansers containing high levels of aggressive surfactant detergents are very challenging for the stability of microcapsules.
Aminoplast microcapsules formed of a melamine-formaldehyde resin have been largely used to encapsulate hydrophobic actives, thus protecting said actives and providing their controlled release. However, capsules such as aminoplast ones suffer from stability problems when used in consumer products comprising surfactants, such as perfumery consumer products, especially after prolonged storage at elevated temperatures. In such products, even though the capsule wall remains intact, the encapsulated active tends to leak out of the capsule by diffusion through the wall due to the presence of surfactants that are able to solubilise the encapsulated active in the product base. The leakage phenomenon reduces the efficiency of the capsules to protect the active and provide its controlled release.
A variety of strategies have been described to improve the stability of oil core-based microcapsules. Cross-linking of capsule walls, with chemical groups such as polyamines and polyisocyanates, has been described as a way to improve stability of microcapsules. WO2011/154893 discloses for instance a process for the preparation of polyurea microcapsules using a combination of aromatic and aliphatic polyisocyanates in specific relative concentrations. Compared to aminoplast, polyurea-based microcapsules present the additional advantage of being free from melamine-formaldehyde. However, these capsules are not always satisfactory in terms of mechanical properties as that are not friable, which can negatively impact their olfactive performance represented by the odor intensity perceived during handling and after intentional breakage e.g. by rubbing.
There is therefore still a need to provide alternative capsules to those known from the prior art, which would be free from melamine-formaldehyde and at the same time olfactively more performing than existing polyurea-based capsules but also as stable as existing melamine-formaldehyde and/or polyurea-based capsules in challenging media such as surfactant-based consumer products.
On the other hand, some prior arts have been describing polyurea-based microcapsules prepared in absence of added polyamine. In particular, WO97/44125 discloses microcapsules prepared by interfacial polymerization process in which polyurea is formed only from an aromatic diisocyanate described as the essential component to form the capsule wall. In this disclosure, an aromatic polyisocyanate having 3 or more isocyanate groups is optionally present but only used as a cross-linking agent. However, although diisocyanates are known to be very reactive and therefore attractive to form a polymeric wall, there are not always considered as suitable component from a safety standpoint. It has been further taught in Single-particle light scattering study of polyethyleneglycol-grafted poly(ureaurethane) microcapsule in ethanol, from Ken Terao et al, in Colloids and Surfaces B: Biointerfaces 37 (2004) 129.132 that capsules based on the use of high levels of triisocyanate to make a capsule wall compared to the quantity of oil to be encapsulated showed poor oil retention upon aging. More particularly, according to this article, even if close to an equiweight of polyisocyanate is used to encapsulate large, high log P molecule (di-2-ethylhexylphtalate) expected to show low propension to leakage through a thick capsule wall, the obtained capsules show poor storage stability when placed in organic solvent, with very high leakage of the (di-2-ethylhexylphtalate) out of the capsule, thus suggesting the polyisocyanate (triisocyanate) used is not suited to yield wall capable of preventing core oil leakage in tough media potentially inducing leakage, be organic solvent or aqueous solutions with high levels of surfactants. Therefore, none of those prior arts is disclosing or suggesting that a performing capsule in terms of perfume retention in challenging medium and good olfactive performance could be obtained based on the use of a polyisocyanate other than a diisocyanate that presents the drawbacks mentioned above.